There are numerous circumstances in which it is desirable to change the field of view of an optical system. For example, numerous applications may benefit from the ability to switch between a narrow field of view and a wide field of view. Referring to FIG. 1, there is illustrated a block diagram of a passive optical imaging system incorporating field of view switching. The system includes an afocal telescope 110 that directs electromagnetic radiation via a pupil 120 to an imager 130. The imager 130 includes various optical components that direct the electromagnetic radiation to a camera system 140, where the electromagnetic radiation is focused onto an imaging detector 142 (such as a focal plane array, for example).
For passive imaging systems, field of view switching is typically provided within the afocal telescope 110 (represented by FOV switch 115). However, in some systems field of view switching may be provided within the imager 130 (represented by FOV switch 135). In passive infrared imaging systems, the imaging detector 142 is typically kept very cold, for example, by placing it within a cryogenic chamber. As a result, the f/# of the optical system is fixed due to the presence of a fixed cold stop 144. Accordingly, changing the field of view of optical system with a field of view switch located in either the afocal telescope 110 (FOV switch 115) or the imager 130 (FOV switch 135) necessarily results in changing the aperture of the afocal telescope, since the f/# is fixed. This is illustrated in FIG. 1, with solid rays 150a, 150b representing the narrow field of view and larger aperture, and dotted rays 155a, 155b representing the wide field of view and smaller aperture. Reduced sensitivity in the wide field of view configuration, due to the smaller aperture, is generally deemed acceptable for passive imaging. If high sensitivity in a wide field of view configuration is desired, the optical system is generally designed for a fixed wide field of view focal length only.